I wouldn't see it degrading the optical quality all that much - besides it's a trade off against not getting the shot at all in the first place.
However, judging from the EF 75-300MM 1:4-5.6III USM vs the EF 75-300mm f/4-5.6 IS USM the IS lens is 170 g heavier and it's also wider and longer by a not inconsiderable amount (7 and 16.2 mm respectively). An adaptor giving similar quality would presumably add similarly to the bulk.
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